Plastic webs such as photographic film bases, that are made by continuous extrusion or melt casting, often exhibit widthwise thickness variations (distribution of thickness across the width of the web) which are persistent in the lengthwise direction. These thickness variations are sometimes called gauge bands or thick/thin streaks. When webs having such gauge bands are wound into rolls, hardstreaks (also called ridges) can form in the winding roll. Hardstreaks are annular bands in the winding roll that are parallel to the sidewall of the roll. Where hardstreaks occur, the diameter of the winding roll is increased, and the pressure between layers in the wound roll is concentrated in this area. Hardstreaks are objectionable because they can lead to web imperfections including: distortions, pressure damage to sensitive coatings and adhesion or blocking of adjacent layers or laps in the wound roll.
To minimize the effect of such thickness variations, both edges of the web can be thickened through an embossing or knurling process and/or the web can be oscillated laterally during winding. Knurling creates artificially thickened areas at the edges of the web which, upon winding, create intentional hardstreaks at the edges. By creating these artificial hardstreaks at the edges in the nonusable portions of the web, a substantial part of the winding tension is used up and the effective tension in the middle portion of the web is significantly reduced, thereby reducing the severity of any hardstreaks which may form in the usable middle part of the web.
Oscillation, as in U.S. Pat. Nos. 2,672,299 and 4,453,659, offsets any thickened portions of the web to reduce the build up of thickness in a particular lateral portion of the wound roll. Although oscillation (also called "wiggle-winding" and "stagger winding") can reduce the development of hardstreaks in the wound web, it can also cause an undesirable amount of edge waste if the oscillation amplitude is large. On the other hand, if the oscillation amplitude is not sufficiently great, the gauge bands in the web are not offset enough to prevent or reduce the formation of hardstreaks.
Although thickening the edges can reduce the hardstreak problem, if they are too thick, i.e., if the "knurl height" is too great, other problems are caused. For example, if the edges are too thick, the web will be supported solely at the thick edges, and buckling will occur in middle of the roll. Also, if all of the roll tension is carried at the edges of the web, the high pressure at the thickened edges can result in "telescoping" or lateral shifting of laps of the roll because of instability in the widthwise direction. Therefore, to reduce the hardstreak problem without creating other problems it is necessary to determine an optimum edge thickness or knurl height for the web.
Similar considerations apply to the tension in the web during winding. Although lowering of tension can reduce hardstreaks, other problems occur if the tension is too low. In particular, at excessively low tension a slippage between layers occurs, a problem known in the art as cinching. Likewise, excessively low tension can cause telescoping or roll shifting.
The described problems can occur in the winding of a wide range of plastic web sizes. The problems are especially serious, however, in the winding of wide plastic webs, e.g., 40 to 80 inches in width, to form large rolls, e.g., of 1.5 to 5 feet in diameter, and especially when the web comprises a thermoplastic film base or support that is coated with one or more photographically sensitive layers and other layers. Such webs are especially susceptible to hardstreak formation, and the waste created by hardstreaks is especially costly. As a consequence, a need exists for a method for controlling the winding of plastic webs so that the severity of hardstreaks in the wound web can be minimized without creating other problems.